Analytical techniques are known in which a high energy plasma of a sample is first formed, which, upon cooling generates a characteristic emission spectrum. More specifically, during this cooling, electrons of atoms and ions at the excited electronic states of the plasma return to their lower energy or ground states, causing the emission of radiation at discreet wavelengths, corresponding to the changes in energy levels. Every element in the periodic table is associated with unique spectral emission lines that may be detected in the visible portion of the electromagnetic spectrum. Emitted light from plasma may therefore be collected and coupled with a spectrograph detector, which can use a diffraction grating to split this light and resolve the emission spectra for the elements in the sample. The intensity of each spectrum can be used to determine the concentration of each associated element.
In laser induced breakdown spectroscopy (LIBS), high temperature plasma formation occurs upon focusing a short-pulse laser beam onto the sample surface, such that a small quantity of the sample is ablated, or removed by both thermal and non-thermal transformations. In optical emission spectroscopy (OES), the discharge plasma is normally formed by the application of electrical energy, such as in the form of a spark generated between an electrode and the sample. OES, however, can also refer to methods using an inductively coupled plasma (ICP) as the excitation source, or otherwise direct-current arc discharge or glow discharge, rather than a spark discharge. Both LIBS and OES provide simple, reliable, and fast methods that are especially suitable for the analysis of metallic samples by resolving their component elements. Concentrations may be accurately determined over a wide range that extends down to parts per million (ppm) levels. For these reasons, LIBS and OES have found widespread applicability in metal-producing industries such as foundries and steel manufacturing.
One particular procedure of importance in the field of LIBS and OES is the analysis of metal wire samples, including weld wire that must consistently meet stringent composition standards to ensure its acceptable performance in fabrication operations. Conventional preparation of a weld wire sample involves using welding equipment to melt the wire, followed by cooling of the molten puddle and grinding to provide the appropriate material for testing. Both LIBS and OES require the sample to be maintained in an inert atmosphere, typically provided by argon gas, within the analysis chamber.